Thermoplastic saturated norbornene resin phase plate

ABSTRACT

A phase plate comprises, as a birefringent layer, a film obtained by stretching and orienting a sheet of a thermoplastic saturated norbornene resin, which has been formed by melt forming. The phase plate of the invention is such that the birefringent layer is optically even at the whole surface, and such optical evenness remains constant even by changes of temperature, humidity and/or the like. The phase plate of the invention can be used in parts for polarization microscopes, parts for liquid crystal displays, etc., and is particularly suitable for use in a phase plate for liquid crystal displays.

TECHNICAL FIELD

The present invention relates to a phase plate, and more particularly toa phase plate, which has, as a birefringent layer, a stretched andoriented film composed of a thermoplastic saturated norbornene resin,and is optically even.

BACKGROUND ART

In order to achieve high definition and large area in liquid crystaldisplay, a high-multiplex operation display in which the twist angle ofliquid crystal molecules was controlled greater than conventional 90degrees has been put to practical use. This is generally called asupertwisted nematic mode (STN-type mode), and an SBE mode, an STN modeand the like are known. In this STN-type mode, steep deformation ofmolecular orientation by application of voltage is combined with theoptical effect of birefringence to optimize retardation (a product ofthe refractive index anisotropy of a liquid crystal and a cell gap=Δn.d)and the azimuthal angle of a polarizer so as to achieve still betterdisplay characteristics. In recent years, black-and-white display hascome to be achieved in the STN mode by a system in which a phase plateis used to compensate for a phase difference of transmitted light, whichhas been caused by the birefringence effect, or the like. It is alsopossible to form full-color images by supplementing color filters, asneeded.

Incidentally, such a phase plate is a plate used in shifting therelative phase of a polarized component of light and made of abirefringent material. In the plate, an oriented film made of asynthetic resin is used as a birefringent layer. Examples of thestructure of the phase plate include a single-layer structure composedof one birefringent layer, a multi-layer structure obtained bylaminating two or more birefringent layers which are identical ordifferent in birefringent behavior, a structure with a protective layer,and the like (for example, Japanese Patent Application Laid-Open No.158701/1990).

In order to provide bright colors and high-definition images, forexample, a phase plate for liquid crystal displays is required that itsbirefringent layer is optically even at the whole surface and undergoesno change in optical properties even by changes of temperature and/orhumidity. In particular, if the phase plate is used in a liquid crystaldisplay panel for installation in a car, it is required to withstand atemperature of at least 60° C. or higher, preferably 80° C. or higher,more preferably 100° C. or higher because it is predicted that the panelwould be used under severe conditions.

As synthetic resin materials for such phase plates, there haveheretofore been used various kinds of film-forming resins such asphenoxyether type crosslinkable resins, epoxy resins, polycarbonateresins, allylate resins and polycarbonate resins.

However, the phenoxyether type crosslinkable resins, epoxy resins,polycarbonate resins, allylate resins and the like are difficult tostretch evenly, and moreover are insufficient in moisture resistance,and hence have hygroscopicity of the order of 0.1-0.2 wt. %. Therefore,their retardation stability is lowered by changes of humidity in aservice environment. Besides, since the polycarbonate resins, allylateresins and the like have high heat resistance, their stretchingtemperature is high. Therefore, it is hard to control the stretchingtemperature, and hence to produce an optically even phase plate.

A film obtained by stretching and orienting a sheet produced by solventcasting is excellent in surface smoothness, but involves problems thatit is poor in productivity, and may not be used according to its serviceenvironment because a solvent remains therein. Therefore, an extrusionprocess making use of a T-die, or the like is used in the polycarbonateresins and the like. However, if a sheet having a thickness of 100-200μm is formed, the unevenness of thickness can be controlled only to theextent of 6-8 μm.

In addition, since these oriented, synthetic resin films generally havea photoelastic coefficient as great as 50-100×10⁻¹³ cm² /dyne, there isa problem that their retardation values are changed greatly by a slightstress. Besides, the unevenness of thickness which occurs in the sheetsbefore stretching directly influences the scattering of retardationafter stretching.

As described above, the phase plates composed individually of theoriented, synthetic resin films conventionally known are not fullysatisfactory for liquid crystal displays. There is hence demand forimproving them.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a phase plate forliquid crystal displays, in which a birefringent layer is optically evenat the whole surface, and such optical evenness remains constant even bychanges of temperature, humidity and/or the like.

The present inventors have carried out an extensive investigation with aview toward overcoming the above-described problems involved in theprior art. As a result, it has been found that an oriented film obtainedby stretching a sheet of a thermoplastic saturated norbornene resinformed by melt forming has excellent properties for a phase plate forliquid crystal displays. The present invention has been led tocompletion on the basis of this finding.

According to the present invention, there is thus provided a phase platecomprising, as a birefringent layer, a film obtained by stretching andorienting a sheet of a thermoplastic saturated norbornene resin, whichhas been formed by melt forming.

BEST MODE FOR CARRYING OUT THE INVENTION

Features of the present invention will hereinafter be described indetail.

Thermoplastic saturated norbornene resin:

The thermoplastic saturated norbornene resins useful in the practice ofthe present invention are known resins from Japanese Patent ApplicationLaid-Open Nos. 14882/1991, 122137/1991, 63807/1992, etc. Specificexamples thereof may include ring-opened polymers of a norbornenemonomer, hydrogenation products thereof, addition polymers of anorbornene monomer, addition polymers of a norbornene monomer and anolefin, and the like.

The norbornene monomers are also known from the above-describedpublications, Japanese Patent Application Laid-Open Nos. 227424/1990 and276842/1990, etc. Examples thereof may include norbornene,dicyclopentadiene, dimethanooctahydronaphthalene, alkyl-, alkylidene- oraromatic-substituted products thereof, and substitution products ofthese substituted or unsubstituted compounds with a polar group such asa halogen, hydroxyl group, ester group, alkoxy group, cyano group, amidegroup, imide group, silyl group or the like.

More specific examples thereof may include norbornene,5-methyl-2-norbornene, 5-dimethyl-2-norbornene, 5-ethyl-2-norbornene,5-butyl-2-norbornene, 5-ethylidene-2-norbornene and the like, andsubstitution products of these compounds with a polar group such as ahalogen; dicyclopentadiene, 2,3-dihydrodicyclopentadiene and the like;dimethano-octahydronaphthalene, alkyl- and/or alkylidene-substitutedproducts thereof, and substitution products of these compounds with apolar group such as a halogen, for example,6-methyl-1,4:5,8-dimethano-1,4,4a,5,6,7,8,8a-octahydronaphthalene,6-ethyl-1,4:5,8-dimethano-1,4,4a,5,6,7,8,8a-octahydronaphthalene,6-ethylidene-1,4:5,8-dimethano-1,4,4a,5,6,7,8,8a-octahydronaphthalene,6-chloro-1,4:5,8-dimethano-1,4,4a,5,6,7,8,8a-octahydronaphthalene,6-cyano-1,4:5,8-dimethano-1,4,4a,5,6,7,8,8a-octahydronaphthalene,6-pyridyl-1,4:5,8-dimethano-1,4,4a,5,6,7,8,8a-octahydronaphthalene,6-methoxycarbonyl-1,4:5,8-dimethano-1,4,4a,5,6,7,8,8a-octahydronaphthalene,etc.; trimers and tetramers of cyclopentadiene, for example,4,9:5,8-dimethano-3a,4,4a,5,8,8a,9,9a-octahydro-1H-benzoindene,4,11:5,10:6,9-trimethano-3a,4,4a,5,5a,6,9,9a,10,10a,11,11a-dodecahydro-1H-cyclopentaanthracene;and the like.

In the present invention, other ring-opening polymerizable cycloolefinsmay be used in combination within limits not impeding the object of thisinvention. Specific examples of such cycloolefins may include compoundshaving a reactive double bond, such as cyclopentene, cyclooctene and5,6-dihydrodicyclopentadiene.

The thermoplastic saturated norbornene resin useful in the practice ofthe present invention has a number average molecular weight in a rangeof generally 25,000-100,000, preferably 30,000-80,000, more preferably35,000-70,000 as determined by gel permeation chromatography (GPC)making use of toluene as a solvent. If the number average molecularweight would be too low, a resin deteriorated in mechanical strengthwill be provided. If the number average molecular weight would be toohigh, easiness of operation upon the synthesis of the resin, and theformation and stretching of a sheet will become poor.

It is desired that the thermoplastic saturated norbornene resin used inthe present invention should have a glass transition temperature (Tg) ofgenerally 90° C. or higher, preferably 110° C. or higher, morepreferably 130° C. or higher.

If the thermoplastic saturated norbornene resin is obtained byhydrogenating a ring-opened polymer of a norbornene monomer, itshydrogenation rate is generally controlled to at least 90%, preferablyat least 95%, more preferably at least 99% from the viewpoint ofresistance to deterioration by heat, resistance to deterioration bylight, and the like.

The thermoplastic saturated norbornene resin is excellent intransparency, heat resistance, moisture resistance, chemical resistanceand the like. In particular, a resin having hygroscopicity of generallyat most 0.05%, preferably at most 0.01% can be provided with ease.Further, its photoelastic coefficient is as small as 3-9×10⁻¹³ cm²/dyne. Such a resin is hence a material suitable for use in producing anoptically even oriented film.

The thermoplastic saturated norbornene resin used in the presentinvention may be added with various additives such as an age resistorsuch as phenolic or phosphoric, an antistatic agent and an ultravioletstabilizer if desired.

Sheet:

The oriented film useful in the practice of the present invention isproduced by first forming the thermoplastic saturated norbornene resininto a sheet and then stretching and orienting the sheet.

In the present invention, the sheet is formed by a melt forming.Examples of the melt forming may include melt-extrusion processes suchas a process making use of a T-die and an inflation process, calenderingprocesses, hot pressing processes and injection-molding processes. Ofthese, the melt-extrusion process making use of a T-die is preferred,which the process can make the unevenness of thickness small, is easy toform the resin into a sheet having a thickness of about 50-500 μm, andcan control the absolute value of retardation and scattering thereofupon the formation of a sheet.

The conditions of the melt forming are substantially the same as thoseused in polycarbonate resins having a Tg similar to the norborneneresin. For example, in the melt-extrusion process making use of a T-die,it is preferable to select the conditions in which the temperature ofthe resin is controlled to about 240°-300° C. and the temperature oftake-off rolls is controlled to a relatively high temperature of about100°-150° C., whereby the resin can be slowly cooled. Besides, in orderto minimize surface defects such as die lines, the die is required tohave a structure in which residence portions are lessened to the utmost,and it is preferable to use a die free of any flaws or the like on itsinterior or lip as much as possible.

These sheets may also be subjected to surface polishing, as needed, tomore increase surface precision.

The thickness of the sheet before stretching is generally controlled toabout 50-500 μm. It is preferred that the unevenness of thickness besmaller. The unevenness of thickness may fall within a range of ±8%,preferably ±6%, more preferably ±4% as a whole. The absolute value ofthe retardation of the sheet may be 50 nm or smaller, preferably 30 nmor smaller, more preferably 20 nm or smaller at the whole surface. Thegreater the unevenness of thickness of the sheet, the wider thescattering of retardation of the stretched and oriented film.

Stretched and oriented film:

The stretched and oriented film useful in the practice of the presentinvention is obtained by uniaxially stretching the sheet. Molecules ofthe film are oriented by the stretching. The thus-obtained stretched andoriented film has a certain retardation value. Incidentally, theuniaxial stretching may be of substantially uniaxial stretching, forexample, biaxial stretching in which a sheet is stretched within limitsnot influencing the molecular orientation of the film, and thenstretched in a uniaxial direction to orient molecules of the film.

The draw ratio is 1.3-10 times, preferably 1.5-8 times. It is onlynecessary to stretch the sheet at a draw ratio within this range so asto have a predetermined retardation value. If the draw ratio is too low,the absolute value of retardation does not increase to fail to obtainthe predetermined value. If the draw ratio is too high, the sheet to bestretched may be broken in some cases.

The stretching is generally conducted in a temperature range of from Tgof the resin forming the sheet to (Tg+50)°C., preferably from Tg to(Tg+40)°C. If the stretching temperature is too low, the sheet isbroken. If the temperature is too high, the resulting film does notundergo molecular orientation. It is hence impossible to obtain thedesired phase plate.

The thus-obtained film in which molecules have been oriented by thestretching has a retardation value of a certain level. In order to usethe stretched and oriented film in a phase plate, the film must have anabsolute value of retardation of 30-1000 nm, preferably 50-800 nm asdetermined at a wavelength of 550 nm. As necessary for its purpose, thefilm is adjusted so as to have the desired retardation value within theabove range. The retardation can be controlled by the retardation of thesheet before the stretching, the draw ratio, the stretching temperatureand the thickness of the stretched and oriented film. If the sheetbefore the stretching has an even thickness, there is a tendency for theabsolute value of retardation of the film to increase as its draw ratiois high. Therefore, it is possible to obtain a stretched and orientedfilm having the desired retardation by changing the draw ratio.

The scattering of retardation value is preferably as narrow as possible.With respect to the stretched and oriented films according to thepresent invention, the scattering of retardation at the wavelength of550 nm is as narrow as a range of generally from -50 nm to +50 nm,preferably from -30 nm to +30 nm, more preferably -20 nm to +20 nm.

The in-plane scattering of retardation and the unevenness of thicknessof the film can be lessened by using a sheet of which the scattering andunevenness are small, and moreover by stretching the sheet under evenstress. In order to conduct such stretching, it is desirable that thesheet should be stretched under even distribution of temperature,preferably in an environment in which the temperature has beencontrolled within a range of ±5° C., more preferably ±2° C., mostpreferably ±0.5° C.

Application:

Applications of the phase plates according to the present inventioninclude parts for polarization microscopes, parts for liquid crystaldisplays, etc.

Phase plate for liquid crystal displays:

The basic structures of the phase plates according to the presentinvention when used as phase plates for liquid crystal displays include(1) a structure having a birefringent layer made of a single layer ofthe stretched and oriented film, and (2) a multi-layer structure havinga birefringent layer made of a plurality of birefringent films includingtwo or more of the stretched and oriented films. In the case of themulti-layer structure, the birefringent films may be stuck together withtheir optic axes of wave normal aligned with each other, and besidesstuck together in such a manner that their optic axes form a certainangle as necessary for its purpose. For example, when a plurality ofstretched and oriented films whose retardation values vary are laminatedwith their optic axes aligned in the same direction, the additiveproperties of the retardation are made good use to obtain a multi-layerfilm having various retardation values. The number of layers laminatedis about 2-6. Examples of an adhesive used in the lamination may includeemulsion adhesives, ultraviolet-curing adhesives, thermosettingadhesives, hot-melt adhesives, etc.

Other structures of the phase plates for liquid crystal displaysaccording to the present invention include (3) a structure in which anoptically isotropic protective layer (for example, an opticallyisotropic polycarbonate film or the like) is laminated on at least oneside of the birefringent layer, (4) a sticker-type structure in which arelease sheet is laminated on at least one side of the birefringentlayer or the optically isotropic protective layer through apressure-sensitive adhesive layer (for example, an acrylicpressure-sensitive adhesive layer) (which can be easily stuck on aliquid crystal cell or the like by peeling the release sheet), (5) astructure in which a phase plate is integrally laminated on a polarizingplate into a phase plate-cum-polarizing plate, and the like.

The stretched and oriented films showing birefringent behavior andcomposed of the thermoplastic saturated norbornene resin are not onlyresistant to temperature changes, but also excellent in moistureresistance and water resistance. In the conventional liquid crystaldisplays, a moisture-resistant and water-resistant protective layer hasbeen often provided to protect an operating liquid crystal cell asneeded. However, sufficient moisture resistance and water resistance canbe achieved without providing at least one of such protective layerswhen the phase plate for liquid crystal displays according to thepresent invention is used, resulting in simplification of structure.

EXAMPLES

The present invention will hereinafter be described more specifically bythe following Referential Example, Example and Comparative Example.However, it should be borne in mind that this invention is not limitedto and by these examples. Incidentally, all designations of "part" or"parts" and "%", which will appear in the following examples, mean partor parts by weight and wt. % unless expressly noted.

The following methods were followed for the measurement of the physicalproperties in the following examples.

(1) The number average molecular weight was measured by GPC making useof toluene as a solvent.

(2) The hydrogenation rate was determined by ¹ H-NMR.

(3) The glass transition temperature (Tg) was measured by using, as asample, a part of a sheet before stretching in accordance with DSC.

(4) The retardation value was determined by means of a Berek compensatorat a wavelength of 550 nm.

(5) The thickness of sheet and film was measured by means of a dialthickness gauge.

(6) The light transmittance was determined in the following manner. Thewavelength of light to be caused to pass through a sheet beforestretching or a stretched and oriented film was continuously changedwithin a range of 400-700 nm to measure a light transmittance at eachwavelength by means of a spectrophotometer. The minimum transmittance inthis measurement was regarded as a light transmittance for the sheetbefore stretching or the stretched and oriented film.

Comparative Example 1

A polycarbonate resin (Lexan 131-111, trade name, product of GE Co.) wasmelt-extruded through a T-die 300 mm wide by means of a single screwextruder equipped with a full-flighted screw 40 mm long, and theresulting extrudate was taken up by chill rolls composed of three rolls300 mm across, thereby forming a sheet. At this time, the temperature ofthe resin in the die was 285° C., and the temperatures of the first,second and third chill rolls were 120° C., 120° C. and 100° C. in thatorder.

Since both side edges of the thus-cooled extrudate became uneven inthickness, the portions were trimmed by 20 mm in width to obtain a sheetbefore stretching, which had a Tg of 141° C., an average thickness of120 μm, an unevenness of thickness of ±8 μm, an average retardationvalue of 15 nm and an in-plane scattering of retardation value of ±20nm.

The thus-obtained sheet before stretching was uniaxially stretched at adraw ratio of 1.5 times under control at 155±3° C. to obtain a stretchedfilm. The film had an average thickness of 80 μm, an unevenness ofthickness of ±6 nm, an average retardation value of 550 nm and anin-plane scattering of retardation value of ±90 nm.

The stretched film was held at 80° C. for 2 hours and then cooled toroom temperature to determine a retardation value. The absolute value ofretardation was found to be 540 nm on the average. The rate of changewas 2% compared with the film before holding at 80° C.

Referential Example 1

To 6-methyl-1,4,5,8-dimethano-1,4,4a,5,6,7,8,8a-octahydronaphthalene(hereinafter abbreviated as "MTD"), were added, as a polymerizationcatalyst, 10 parts of a 15% solution of triethylaluminum in cyclohexane,5 parts of triethylamine and 10 parts of a 20% solution of titaniumtetrachloride in cyclohexane, thereby conducting ring-openingpolymerization in cyclohexane. The thus-obtained ring-opened polymer washydrogenated with a nickel catalyst to obtain a polymer solution. Thispolymer solution was solidified in isopropyl alcohol, and the resultingsolids were dried to obtain a resin in the form of powder. The resin hada number average molecular weight of 40,000, a hydrogenation rate of atleast 99.8% and a Tg of 142° C.

Example 1

The powdery resin obtained in Referential Example 1 was melted at 250°C. to form pellets. These pellets were used to produce a sheet in thesame manner as in Comparative Example 1. At this time, the temperatureof the resin in the die was 275° C., and the temperatures of the first,second and third chill rolls were 120° C., 120° C. and 100° C. in thatorder.

Since both side edges of the thus-obtained sheet before stretchingbecame uneven in thickness, the portions were trimmed by 20 mm in widthto observe the surface of the sheet visually and through an opticalmicroscope. As a result, none of blowing, streaks, marks and the likewere recognized. The sheet had a Tg of 139° C., an average thickness of150 μm, an unevenness of thickness of ±4 μm or smaller, a lighttransmittance of 90.5%, retardation values of 22 nm on the average andof 22 nm in the plane, and an in-plane scattering of retardation valueof ±10 nm.

The sheet before stretching was uniaxially stretched at a draw ratio of2.5 times under control at 140°±2° C. to obtain a stretched and orientedfilm.

The stretched and oriented film had an average thickness of 55 μm, anunevenness of thickness of ±1.5 μm, an average retardation value of 580nm and an in-plane scattering of retardation value of ±20 nm.

The stretched and oriented film was held at 80° C. for 2 hours and thencooled to room temperature to determine a retardation value. It wasfound to be 575 nm on the average. The rate of change was 1% comparedwith the film before holding at 80° C. Therefore, this stretched andoriented film was excellent in retardation stability to temperaturechange compared with the film made of the polycarbonate, and was narrowin in-plane scattering of retardation value.

Industrial Applicability

According to the present invention, there are provided phase platesoptically even and excellent in heat resistance and moisture resistance.The phase plates of the present invention can be used in parts forpolarization microscopes, parts for liquid crystal displays, etc.

We claim:
 1. A phase plate comprising, as a birefringent layer, a film obtained by uniaxially stretching and orienting a sheet of a thermoplastic saturated norbornene resin at a draw ratio within a range of 1.3-10 times, wherein the thermoplastic saturated norbornene resin has a number average molecular weight ranging from 25,000 to 100,000 and a glass transition temperature of 90° C. or higher, and the sheet is formed from the thermoplastic saturated norbornene resin in accordance with a melt-extrusion process making use of a T-die and has a thickness ranging from 50 to 500 μm.
 2. The phase plate as claimed in claim 1, wherein a retardation value of the stretched and oriented film at a wavelength of 550 nm falls within a range of 30-1000 nm.
 3. The phase plate as claimed in claim 1, wherein an in-plane scattering of retardation value of the stretched and oriented film at a wavelength of 550 nm falls within range of from -50 nm to +50 nm.
 4. The phase plate as claimed in claim 1, wherein the birefringent layer has a multi-layer structure in which at least two stretched and oriented films have been laminated on each other.
 5. The phase plate as claimed in claim 4, wherein the multi-layer structure is such that at least two stretched and oriented films are laminated with their optic axes aligned in the same direction.
 6. The phase plate as claimed in claim 1, wherein an optically isotropic protective layer is laminated on at least one side of the birefringent layer.
 7. The phase plate as claimed in claim 1, wherein a release sheet is laminated on at least one outermost side of the birefringent layer through a pressure-sensitive adhesive layer.
 8. The phase plate as claimed in claim 1, wherein a polarizing plate is integrally laminated thereon.
 9. The phase plate as claimed in claim 1, which is a phase plate for liquid crystal displays. 